152 



HARDWICKE'S SCIENCE-GOSSIP. 



[July 1, 1870. 



others, again, they are reduced to mere protuber- 

 ances from the carapace. These appendages are 

 ordinarily awl-shaped and pointed ; occasionally they 

 are cylindrical and obtuse ; and in some species they 

 are flattened, and even have their edges truncated. 



In one species, which inhabits New Holland, M. 

 Hupc found a mollusc Gasteropod, belonging to the 

 genus Stylifera, enclosed in one of the spines, which 

 was hollowed, and greatly changed, both in form 

 and structure, by the presence of this little parasite. 



Among all the sights which Nature presents to 

 us, there is scarcely one more interesting than that 

 of creatures giving to each other shelter, and food, 

 and protection, whether voluntarily or involun- 

 tarily. Is not the instinct of the Stylifer marvel- 

 lous ? Nature has bestowed upon one creature an 

 armour of bristling bayonets, when another animal, 

 much smaller, seems to approve of the admirable 

 defence, and takes up its abode in the midst — nay, 

 actually in the spines, which henceforth protect 

 itself as well as the urchin. 



When the bristles fall off, the echini are found on 

 our shores, very much like round fruit, ornamented 

 on the sides with tubercles, symmetrically arranged. 

 Their round form, and, perhaps, especially the limy 

 nature of their shells, has obtained for them the 

 name of Sea-eggs. The flattened species, denuded 

 of their spines, are more like cakes than eggs. 



The tentaculse of the sea-urchins are hollow, very 

 elastic, and are terminated by a sucker. The ani- 

 mals can inflate them by injecting into them liquid 

 through their prickles, and by this means they can 

 fix themselves to any foreign body. These organs 

 are very numerous ; in the ordinary urchin there 

 are at least 1,400, and in the Melon Echinus about 

 4,300. They can move by means of their tentaculae 

 and their spines. Professor Edward Forbes once 

 saw one crawl up the sides of a very slippery vase. 



To understand better how they use their organs 

 of locomotion, imagine one at rest. All the spines 

 are motionless, all the filaments are contracted 

 within the shell; when the creature wishes to move, 

 some of these involuntarily begin to come out ; they 

 extend themselves, and feel the ground all round 

 them; then others follow. The animal fixes some 

 of its tentacles to the vase in the direction in which 

 it wishes to advance, these then contract, while the 

 hinder ones loosen their hold, and thus the shell is 

 drawn forward. The sea-urchin can thus advance 

 with ease and even rapidity. During the progres- 

 sion, the suckers are only slightly aided by the 

 spines ; indeed, the latter only serve as points, upon 

 which the creature rolls as if it were on stilts. It 

 can travel as well on its back as on its stomach. 

 Whatever may be its posture it has always a certain 

 number of spines which arc ready to carry it, and 

 suckers which can fix it. In certain circumstances, 

 the animal walks by .turning itself round on its 

 spines, like a wheel in motion. 



The mouth of the Echinus is situated underneath 

 and is generally in the centre. Around this orifice 

 are fleshy tentacles, projecting from the surface, 

 and more or less retractile. These are the organs 

 which seize the food. 



Fig. 136. Magnified larva of Echinoderm PCtiteus paradoxus. 



The digestive system presents a very complicated 

 osseous apparatus, for a long time known as "Aris- 

 totle's lantern." It consists of five pieces — the 

 teeth, the plumula, the pyramids, the compass, and 

 the scythe. 



Fig. 137. The Buccal apparatus of an Echinus, magnified. 

 ' ' Aristotle's lantern. ' ' • 



The teeth are five in number. T'aey are fixed on 

 the same base, which is the plumuZla, and these are 

 situated upon the edge formed by t^he assemblage of 

 the pyramids, which are ten in .number, and are 

 joined in pairs. The lower part iss made firm by the 

 five scythes, and the five compasses. In fine, the 

 dental apparatus consists of f no less than thirty 

 pieces. The teeth are long, sharp, curved, and very 

 hard. They can cut the hardesjt substances. How- 

 ever, in spite of their adamantine character, they 

 would soon be worn down by work-, but Nature has 

 wisely provided for their renewal. "..They grow from 

 the base as they are worn down at tin*? points, like 

 the incisors of beavers, hares, or rats : si i that they 



